1. Field of the Invention
The present invention relates to an improved system and method for stirring suspended solids in a liquid media. More particularly, the present invention relates to a system and method employing a stirrer, in particular, a magnetic ferrous metal-filled polymer, which is deposited in a vessel containing a liquid media that includes a suspended solid, and is manipulated by a moving magnet outside the vessel to stir the suspended solid in an optimal manner.
2. Description of the Related Art
Many medical diagnoses require that a fluid sample, such as a blood sample, be taken from a patient, cultured in a growth medium, and then examined for the presence of a pathogen believed to be causing the patient's illness. The growth medium provides nutrients that allow the pathogen, such as bacteria, virus, mycobacteria, mammalian cells or the like, to multiply to a sufficient number so that their presence can be detected.
In some cases, the pathogen can multiply to a large enough number so that it can be detected visually. For example, a portion of the culture can be placed on a microscope slide, and visually examined to detect for the presence of a pathogen of interest.
Alternatively, the presence of a pathogen or other organism can be detected indirectly by detecting for the presence of byproducts given off by the microorganism during its growth. For example, certain microorganisms such as mammalian cells, insect cells, bacteria, viruses, mycobacteria and fungi consume oxygen during their growth and life cycle. As the number of microorganisms increases in the sample culture, they naturally consume more oxygen. Furthermore, these oxygen consuming organisms typically release carbon dioxide as a metabolic byproduct. Accordingly, as the number of organisms present increases, the volume of carbon dioxide that they collectively release likewise increases.
Alternatively, instead of detecting for the presence of carbon dioxide to detect the presence of an oxygen consuming microorganism, it is possible to detect for a depletion in the concentration of oxygen in the sample of interest. The presence of oxygen consuming organisms can also be detected by detecting for a change in pressure in a sealed sample vial containing the sample of interest. That is, as oxygen in a closed sample vial is depleted by oxygen consuming organisms, the pressure in the sealed sample vial will change. The pressure will further change in the sample vial as the organisms emit carbon dioxide. Therefore, the presence of such organisms can be detected by monitoring for a change in pressure in the closed sample vial.
Several methods exist for detecting the presence of carbon dioxide in a sample to determine whether organisms are present in the sample. For example, an instrument known as the BACTEC® 9050 manufactured by Becton Dickinson and Company detects for the change in color of an indicator to determine whether carbon dioxide is present in a sample. That is, each sample is collected in a respective sample vial containing an indicator medium having a chemical that reacts in the presence of carbon dioxide to change color. A light sensor detects the color of the indicator medium in the sample vial when the sample vial is loaded into the instrument. If the sample contains an organism which emits carbon dioxide, the reflected or fluorescent intensity of the indicator medium will change in response to the presence of carbon dioxide. The light sensor will therefore detect this change in intensity, and the instrument will thus indicate to an operator that an organism is present in the sample contained in the sample vial. Other examples of instruments for detecting the presence of organisms in a sample by detecting for the change in carbon dioxide in the sample are described in U.S. Pat. Nos. 4,945,060, 5,164,796, 5,094,955 and 5,217,876, the entire contents of each of these patents are incorporated herein by reference.
An instrument employing an oxygen detecting technique is described in U.S. Pat. No. 5.567,598, the entire content of which is incorporated herein by reference. Instruments that are capable of detecting changes in pressure in the sample vial are described in U.S. Pat. Nos. 4,152,213, 5,310,658, 5,856,175 and 5,863,752, the entire contents of each of these patents are incorporated herein by reference. In addition, an instrument capable of detecting changes in carbon dioxide concentration, changes in oxygen concentration, and changes in pressure in the vessel is described in a U.S. patent application of Nicholas R. Bachur et al. entitled “System and Method for Optically Monitoring the Concentration of a Gas, or the Pressure, in a Sample Vial to Detect Sample Growth”, Ser. No. 09/892,061, filed on Jun. 26, 2001, and another instrument capable of detecting changes in carbon dioxide concentration or changes in oxygen concentration in the vessel is described in a U.S. patent application of Nicholas R. Bachur et al. entitled “System and Method for Optically Monitoring the Concentration of a Gas in a Sample Vial Using Photothermal Spectroscopy to Detect Sample Growth”, Ser. No. 09/892,012, filed on Jun. 26, 2001, the entire contents of both of said applications being incorporated herein by reference.
It is noted that the results obtained by organism detection techniques described above can be improved if the growth of the organism is enhanced to cause a greater production of carbon dioxide, a greater depletion of oxygen, and a greater change in pressure in the vessel. It is known that the biological activity of a solid sample in a liquid media can be enhanced by maintaining the solid sample in a suspended state. This can be accomplished by continuously stirring the solid-liquid mixture, which improves nutrient, waste and gas exchange in the mixture.
Examples of stirring techniques are described in U.S. Pat. Nos. 5,586,823, 4,483,623 and 4,040,605, the entire contents of each are incorporated herein by reference. Each of these techniques employs a magnetic stirrer that is placed in the vessel containing the sample and manipulated by a magnet to stir the sample in the vessel.
Although these stirring techniques may be somewhat effective in enhancing sample growth, they each suffer from certain disadvantages. For example, because each of the techniques require that the vessel be maintained in a vertical configuration, the fluid-gas interface is minimized, especially in vessels that are not shallow. This minimal fluid-gas interface inhibits biological performance in the vessel.
In addition, the vertical configuration of the vessel allows for the magnets to lose their influence over the magnetic stirrer in the vessel, especially if the magnetic influence on the stirrer is weak as in the case of gentle stirring. Also, the vertical configuration causes the stirrer in the vessel to follow a semi-random stirring path, which results in a stirring action that is inefficient and potentially damaging to the sample. Furthermore, in order to change the intensity of the stirring in these known arrangements, the physical size of the stirrer or the apparatus needs to be changed.
A need therefore exists for an improved system and method for stirring suspended solids in a liquid media to enhance sample growth and thus improve sample detection results.